1. Field of the Invention
This invention relates generally to X-ray tubes, and more particularly relates to dual focus X-ray tubes having a pair of simultaneously energized cathode filaments and to methods of dual filament tube operation.
X-radiation is used in a variety of examining procedures in diagnostic medicine. With these procedures a patient is positioned between an X-ray tube head and a detector, such as a sheet of X-ray film or a fluoroscopic device. An X-ray tube in the tube head emits a beam of X-rays which passes through the patient and impinges upon the detector. The X-ray beam produces, on the detector, a shadow image which is indicative of the condition of the internal structure of the patient and is used in the diagnosis of the condition of the patient.
In these examining procedures, the X-ray beam is desirably of uniform intensity in a plane which is normal to the beam axis. As the intensity becomes more uniform, the resolution, the amount and the quality of information obtained for diagnostic purposes increases. Accordingly, the X-ray tube should be structured and operated to produce an X-ray beam having as uniform intensity as possible.
X-ray tubes have an anode structure which defines a target. One or more cathodes, usually thermionic filaments, are provided for supplying electrons. A positive operating potential is applied between the cathode and anode structures for causing electrons released by the filaments to flow to and impinge upon the target. The target emits a beam of X-rays in response to the electrons.
The target is a surface positioned at an obtuse angle with respect to the path of electron flow. The electron-emitting area of the target is preferably configured in the shape of a rectangle which, due to the angular orientation of the target, appears as a square when viewed along the axis of the X-ray beam. Bombardment of this target area with appropriately distributed electrons produces an X-ray beam of uniform intensity which is capable of producing an image of high resolution. Ideally, an X-ray tube should be structured to produce a flow of electrons from the cathode structure to the target of the anode structure such that the target is bombarded with the electrons which are distributed uniformly, both longitudinally of and transversely of the target.
2. The Prior Art
There have been many prior proposals for overcoming the problems encountered in producing a uniformly distributed electron flow with a single filament X-ray tube. One such problem is that electron emission from a heated filament is uneven because the typical filament is unevenly heated with the center portion of the filament being heated to a greater extent than the end portions. There have been a number of proposals for overcoming, or at least limiting the effects of uneven electron emission. Some of these proposals have suggested a specially configured or constructed filament. A widely accepted and at least a partial solution to this problem is the use of an electrically biased focusing cup. Such a cup greatly improves the uniformity of electron flow longitudinally of the target by providing a mechanism for electrostatically focusing electrons. The electron optic characteristics of a focusing cup do not produce uniform distribution transversely of the target. Rather, electron impingement on the target is concentrated in two long, thin spaced and parallel regions. This focusing cup method of tube operation is described in detail in the above-referenced FOCAL SPOTS patent.
Another problem encountered with a hot filament X-ray tube is that of the so-called space charge limitation. As electrons are emitted from a heated filament, a space charge develops near the filament due to a residual accumulation of electrons which do not travel to the anode. Generally as more electrons are emitted from the filiment, the space charge increases for a given operating potential between the filament and the anode. One prior attempt at controlling the space charge has used an extra element, a grid electrode, in the X-ray tube. A positive voltage applied to the grid electrode tends to attract some of the residual electrons to reduce the space charge and allow increased electron flow. This technique is described in greater detail in the SPACE CHARGE patent.
Another limitation on the performance of an X-ray tube is the maximum temperature at which a hot filament may be properly operated. If the filament is operated in excess of its rated temperature, the structure of the filament is eventually weakened, substantially shortening the life of the filament and the tube. Because a thermionic filament produces electrons in proportion to its temperature, the temperature limitation inherently limits the production of electrons. Further, if a filament is heated beyond its rated temperature, it may distort and adversely affect the electron optic characteristics of the tube.
There have been prior attempts to produce electron flow of the desired characteristics in double filament X-ray tubes. These attempts have concurrently energized both filaments to provide two flows of electrons which impinge upon the target concurrently. The respective flows of electrons are focused either in nonoverlapping, laterally adjoining focal spot regions or in a wholly overlapping focal spot pattern. In either case, both flows of electrons strike the anode in patterns which together define a single X-ray-emitting target area.
In the nonoverlapping prior proposal, the combined focal spots are of a relatively large size and are bombarded by heavy flows of electrons to produce an X-ray beam of relatively high intensity. X-ray beams of high intensity are used in examination procedures, such as radiography, which require relatively large dosages of X-radiation. The individual heavy flows of electrons strike the target in radially transverse line focus patterns. Because the patterns were nonoverlapping, the overall electron distribution striking the target was nonuniform.
In the wholly overlapping pattern prior proposal, a smaller line focus-type focal spot was wholly contained within a large line focus-type spot. In this type of tube, two filaments and associated focusing mechanisms of differing configurations were required to produce the larger, outer and the smaller, inner spots. The differing configurations undesirably complicated the manufacturing requirements needed to produce a tube having an overall focal spot of uniform electron distribution.
Prior art, dual filament, X-ray tubes generally excited the filaments concurrently by connecting them to a common power source through separate filament transformers of hopefully equal output. Each filament transformer produced nonadjustable operating voltage to its associated filament. Theoretically, ideal and identical filaments would produce a pair of electron flows of the same distributional characteristics with this type of energizing circuit. As a practical matter, filaments are not ideal and identical. Even when similarly energized, each filament emits a slightly different electron distribution than the other filament pattern. Accordingly, prior dual filament tubes which utilized concurrent energization could not dependably provide, nor were they individually adjustable to provide, a pair of similarly distributed flows of electrons to the target area.
Although separately adjustable filament transformer circuits in dual filament tubes which do not concurrently energize the filaments are known, it has not been known that extraordinary improvements in emission uniformity can be obtained by using separately adjustable filament transformer circuits in concurrently energized dual filament tubes.
Anode heat generation is another consideration in X-ray tube design. As electrons bombard the target, heat is generated. To prolong target life (and thus tube life), the temperature of the target must be kept below its rated value.
To increase the X-ray output for a given target size while not overheating the target, structures have been developed which lower the temperature of the electron-emitting target. X-ray tubes now commonly use a rotating anode which defines a rotating, frustoconical, target. The target is rotated at a high speed about an axis generally paralleling an extension of the axis of electron flow. Only a relatively small portion of the overall target area is within the electron path at a given time. The portions not in the electron path cool by radiation and conduction of the heat away from the target. Thus, the target area is constantly changing and any given portion of the target surface has time to cool as it is intermittently bombarded by the electrons. Accordingly, as compared with a fixed target, the target area in the electron path may be more heavily bombarded with electrons and can be operated at the higher temperature for a short period of time.
Prior rotating anode X-ray tubes have not provided optimum X-ray emitting efficiency for a given target area. As has been indicated, the conventional anode structure is frustoconically shaped with the electrons striking the target area at an angle. This produces an X-ray beam which has an axis transverse to the path of electron flow. For a given speed of anode rotation, radially outward regions of the target are moving at a faster velocity than inner regions. Because of this high rotational speed, the outward regions are bombarded for a shorter time interval than inner regions and are thus able to withstand a heavier electron bombardment without overheating. While the outer regions can withstand higher bombardment than inner regions, the intensity of electron flow has in the past been limited to a level which can be withstood by the radially innermost portions of a target area.